The present invention relates to chairs which can be placed by the user in either task accomplishing positions or rest positions and, more particularly, to such positionable chairs which also position the spine and pelvic region of the user to reduce efforts required of associated muscles.
The posture taken by an individual while seated often determines the amount of strain occurring in the muscles associated with the spine and pelvis, and the fatigue that results. The posture may be poor, either because the chair in use forces the spine into unnatural positions or because the user slouches while sitting.
Typical chair backrest cushions often have a bulge on the user's side of thereof which is located at a relatively low position therealong with respect to the back, often just above the pelvic area. Such cushion bulges block the movement of the supraspinous ligament and several of the vertebral segments. Rearward movement of these segments is necessary for achieving proper equilibrium in the spine of one sitting on a chair in an upright position in accomplishing tasks in front of them. Because such spinal movement is blocked, the center of gravity of the body is kept too far forward leading to a moment about the ischium of the seated pelvis which must be compensated for by various spinal muscles continually contracting.
On the other hand, many chairs do not permit the user to also recline somewhat with the spine tilted significantly backward for purposes of resting, a decided help since rearward inclination of the back on an inclined backrest permits the backrest to support up to half the weight of the upper torso. An inability to incline backwards to permit the chair backrest to take up some of the upper torso weight leaves only the back muscles to continually support such weight.
In doing tasks in an upright sitting position, a user of a chair will have a desire to be tilted relatively forward of, or backward from, some usual task-accomplishing position, and at varying angles. The desired changes from this usual upright position will depend on the particular portion of the task being undertaken. Also, the usual position taken for accomplishing tasks will depend on the task, the length of the person's legs and arms, etc. The ability of the user to adjust his upright position continually at his or her desire to a selected degree will be less tiring for that person if such adjustments can be made while minimizing any efforts which involve having to shift position with respect to the chair, or involve having to move the chair along the horizontal surface on which it rests.
Thus, there is desired a chair which fully supports the natural lumbar lordotic curve of the lower spine of a user in most upright positions and in any reclining positions, the chair being able to permit the user to conveniently attain all such positions. Further, the chair should be able to change between various upright positions easily, and to change between upright positions and reclining positions easily.